Problem with user admin pages for less privileged users

I've implemented the FBA Pack for a client. I've modified the code to put the new users in a specific SharePoint group. Fine. This group uses a custom permission level that has a bit less privileges than the Contribute permission level.

Now, the FBA user admin pages work fine when I'm logged in as a administrator. If I log in with a FBA user, I get an access denied error. I found the
RequireSiteAdministrator property in the source code and changed it to return false rather than true. Now I don't get the access denied error anymore, however the page (UserDisp.aspx) fails miserably. The error log gives me the following: